1. Field of the Invention
The present invention relates to particulate magnetic recording media for high-density recording.
2. Description of Related Art
Recently, there is a tendency toward shorter recording wavelength with higher densification, resulting in the aggravation of the problem of self-demagnetization loss during recording leading to output loss with a thick magnetic layer. Thus, a thinner magnetic layer has been made, but a magnetic layer of 2 xcexcm or less directly applied on a nonmagnetic substrate are susceptible to the influence of the substrate on its surface to deteriorate electromagnetic characteristics or aggravate dropouts.
A possible solution to this problem was to apply an underlying nonmagnetic layer and a thin layer of a concentrated magnetic coating solution thereon by simultaneous multi layer coating, as disclosed in JP-A Nos. 191315/88 and 187418/88. These inventions dramatically improved the yield to attain good electromagnetic characteristics. In order to achieve a further higher density, magnetoresistive reproducing heads (MR heads) are commercialized in hard discs and experimental reports exist on metal-evaporated tapes (ME) in flexible media (IEEE. trans. mag. VOL. 35, No.2, p. 729 (1999)).
Although magnetic recording media suited to reproduction with MR heads on metal-evaporated tapes (ME) have been examined, any particulate magnetic recording media with good productivity or preservability have not been sufficiently examined.
Therefore, it is an object of this invention to provide a particulate magnetic recording medium showing a high C/N in high-density magnetic recording (especially using an MR head for reproducing).
Accordingly, the present invention relates to a magnetic recording medium comprising a lower layer containing a nonmagnetic powder and a binder and a magnetic layer containing a ferromagnetic powder and a binder provided on a nonmagnetic flexible substrate in this order wherein said magnetic layer has an average thickness d in the range of from 0.01 to 0.1 xcexcm, said ferromagnetic powder contained in said magnetic layer is an acicular ferromagnetic alloy powder having an average major axis length equal to or less than 0.1 xcexcm and a saturation magnetization as equal to or less than 120 A.m2/kg (120 emu/g), said nonmagnetic powder contained in said lower layer is an acicular inorganic powder having an average major axis length equal to or less than 0.15 xcexcm, and said ferromagnetic powder has an aspect ratio equal to or smaller than the aspect ratio of said acicular inorganic powder.
Preferred embodiments of the magnetic recording medium of the present invention have the following features.
(1) The above-mentioned ferromagnetic alloy powder has an average particle volume in the range of from 1500 to 15000 nm3 and a coercivity Hc equal to or more than 167 KA/m (2100 Oe).
(2) The above-mentioned ferromagnetic powder in said magnetic layer has a volume packing density equal to or more than 30%.
(3) The above-mentioned lower layer contains a granulated powder having an average particle size equal to or less than 50 nm and a true specific gravity equal to or less than 5 in an amount ranging from 10 to 30 parts per 100 parts of the acicular inorganic powder.